This invention includes a second embodiment of a high energy transient semiconductor protection device disclosed in, and this patent application incorporates by reference all material contained in, U.S. patent application Ser. No. 07/745,621, titled Semiconductor Protection Against High Energy Transients, filed Aug. 15, 1991. Embodiments of the apparatus disclosed and claimed in the referenced patent application constitute certain of the elements of the combination of the present application.
Electrical transient protection devices have been known for many years. For example, U.S. Pat. No. 4,525,765, issued on Jun. 25, 1985 to Brajder, discloses a protective circuit for a switching transistor. The protective circuit includes a comparator that compares the base-emitter voltage of the switching transistor with a reference voltage. Any increase in the collector current of the switching transistor will be reflected by an increase in base-emitter voltage, the base current remaining constant. Whenever the base-emitter voltage exceeds the reference voltage, the comparator generates an output signal. Driving pulses that switch the switching transistor into and out of a state of conduction are controlled by a device responsive to the output signals from the comparator so that the switching transistor is switched off when its collector current increases above a predetermined level, thereby preventing the flow of excessive current through the transistor.
U.S. Pat. No. 4,816,963, issued on Mar. 28, 1989 to Eden, discloses an apparatus for protecting a transistor when an associated load is shorted. The circuit compares a command voltage with a reference voltage; and the transistor is turned on if the command voltage exceeds the reference voltage and is turned off if the reference voltage exceeds the command voltage. The reference voltage is developed across a capacitor that is charged quickly when the load is shorted and is discharged slowly. This keeps the transistor turned on for short periods and turned off for comparatively long periods, the former being short enough to ensure the protection of the transistor while the load is shorted.
U.S. Pat. No. 4,589,049, issued on May 13, 1986 to Krumrein, discloses a protection circuit for integrated circuits. The protection circuit is connected across the integrated circuit to be protected and includes a first transistor in series with a measuring resistor, the series combination thereof also being connected across the integrated circuit. A zener diode, or its equivalent, is connected between the base and collector of the first transistor so that the latter will conduct if an excessive amount of voltage is applied across the integrated circuit.
Whenever the first transistor conducts, a voltage is developed across the measuring resistor. This voltage is compared to a reference voltage by a comparator the output of which is connected to the base of a second transistor, the collector of which is connected to the base of the first transistor. When the voltage is developed across the measuring resistor, the output of the comparator turns on the second transistor, which renders the first transistor more conductive.
With the first transistor turned on, the series circuit of the first transistor and the measuring transistor shunts current flow therethrough, developing thereacross a relatively low voltage, which is also reflected across the integrated circuit to which it is connected in parallel. A limiting resistor is used between a source of operating potential and the first transistor to develop thereacross a major portion of voltage applied across the series circuit including the limiting resistor, the first transistor and the measuring resistor.
U.S. Pat. No. 4,849,845, issued on Jul. 18, 1989 to Schmitt, disclose transient suppressor for dissipating overvoltages developed across a power supply connected to a load. The transient suppressor senses an overvoltage condition with respect to a reference voltage and turns on a transistor connected across the power supply. The conducting transistor and a resistor connected in series with it across the power supply shunt current flow from the power supply, developing a relatively low voltage across the series circuit and across the integrated circuit to which it is connected in parallel, the resistor dissipating excess electrical energy while the transistor is conducting. When the voltage drops to a level below that of the overvoltage, the transistor is turned off. If the overvoltage continues, the transistor is turned on and off repeatedly. The number of transistor transitions are counted; and, if a predetermined number is exceeded, the operation of the transient suppressor is inhibited and the supply of electrical power is discontinued.
The protective circuits disclosed in the first two patents, to Brajder and Eden respectively, are of the type that turn off the transistors to be protected when an electrical transient is detected. The circuits each require added current-sensing, decision and control circuitry and protect single devices.
The last two patents, to Krumrein and Schmitt respectively, provide clamping circuits that shunt current around circuits to be protected. The Krumrein circuit requires an added overvoltage detector, current measuring circuit, clamping transistor and limiting resistor and protects a single device. The Schmitt patent requires an added overcurrent detector, decision and control circuitry, and clamping hardware.
While each of these protection circuits functions with a certain degree of efficiency, none discloses a device for transistor protection against high energy transients wherein so few elements need to be added to the device to be protected, wherein switching transistors can have relatively low voltage ratings, wherein relatively long applications of high energy transients can be tolerated, wherein a number of devices can be protected by one protection device, and wherein the functions of a processing device, or microcomputer, which may be used for other purposes in an associated circuit, can be shared by the protection device as does the present invention as is hereinafter more fully described.